A damper is a hardware device for dissipating energy in a mechanical system having relatively movable parts. A damper provides forces opposing relative motion of the movable parts. Commonly encountered examples of damping apparatus include the shock absorbers in a car and the lead-lag damper in the rotor system of a helicopter.
A variety of damping apparatus are known in the art, including friction dampers and elastomeric dampers, which dissipate energy through the rubbing or deformation of solid materials, and pneumatic and hydraulic dampers, which dissipate energy by pumping fluids through an orifice. A recent variation to the hydraulic-type damper utilizes magnetorheological (MR) fluids comprising magnetic particles suspending in a carrier fluid such as an oil or gel. These MR fluids undergo a change in apparent viscosity in the presence of a magnetic field. Examples of MR fluid dampers are disclosed in the following U.S. Pat. Nos.: 5,277,281; 5,284,330; 5,382,373; 5,398,917; and 5,492,312. MR fluid dampers have the ability to change the apparent viscosity of the working fluid, and thus the damping characteristics of the device, by changing the strength of the magnetic field, for example, by changing the current flow through the coils of an electromagnet.
While MR fluid dampers allow the electrical adjustment of damping characteristics, they also have the following disadvantages: first, the fluid component of an MR damper can leak out of the device if the integrity of the sealed cavity is not maintained, degrading the performance of the damper and possibly contaminating other system components with the abrasive fluid. Second, the magnetic particle component of an MR fluid will "settle out" of the fluid component over time or when exposed to high G-forces, i.e., those over approximately 10 G's. Third, the fluid component of MR fluid will generally change viscosity as a function of temperature, and may even freeze or vaporize at temperature extremes such as those that may be encountered in aircraft applications where components may be exposed to temperatures ranging from 55.degree. C. during high temperature operation down to -45.degree. C. when stored under arctic conditions. Fourth, MR fluids are highly abrasive due to the small particles contained in the carrier fluid. This abrasive quality will tend to erode orifices through which MR fluids are pumped during the damper operation, and will also erode dynamic seals or other sliding surfaces.
Devices are also known which utilize the adhesion of dry magnetic particles to transmit forces between rotating members. For the purposes of this application, particles are considered "dry" when they are not suspended or immersed in a liquid or gel medium. Examples of such devices are the well known magnetic particle clutch and the magnetic particle brake. A magnetic particle clutch typically consists of a first rotating (input) shaft connected to a magnetic disk and a second rotating (output) shaft connected to another magnetic disk. These disks have a small gap between them and the gap is filled with a finely divided magnetic powder. Both disks and the gap are contained within a magnetic housing which also contains an electromagnetic coil. When electric current passes through the coil, it establishes a magnetic field in the gap and the two magnetic disks. This magnetic field causes the magnetic particles to adhere to one another and the adjoining disks and form chains bridging the gap between the two disks such that torque is transmitted between the two rotating shafts. Magnetic particle brakes are similar except that the output shaft is attached to a non-rotating "ground," or is replaced by part of the housing, which is "attached to ground." Magnetic particle clutches and brakes are known in which the magnetic field is produced by either a permanent magnet or an electromagnet. Where a permanent magnet is used the clutch will transmit torque between the rotating input and output shafts until a maximum "slip" torque is achieved, at which time the input shaft will begin to slip with respect to the output shaft, however, the clutch will continue to transmit torque between the shafts at the slip torque value. When the magnetic field of a magnetic particle clutch is provided by an electromagnet, it is possible to have an intermittent-acting clutch by turning the electric current through the coils of the electromagnet on or off, or alternatively, it is possible to have a magnetic clutch in which the slip value of torque transmission may be varied by varying the electric current passing through the coils. Whether using permanent magnets, electromagnets, or a combination of both, however, magnetic particle clutches have always been used to transmit torques between rotating shafts or to limit the maximum torque transmitted through a system by allowing rotation between shafts.